1. Field of the Invention
The present invention relates to an evaporator for use in a car air conditioner, and more particularly relates to a stack type evaporator which is improved not to cause the condensed waterdrop to fly and to be free from the problem of bad smell.
2. Description of the Prior Art
The stack type evaporators of this kind usually comprise plate-like tubular elements each composed of a pair of dish-like core plates 60 as shown in FIG. 11. These core plates face one another and are adjoined at their peripheries 60a. Such tubular elements are stacked side by side in the direction of thickness, with fin members each being interposed between the adjacent tubular elements. An inlet and outlet header portions 60b and 60c are formed at an end of each tubular element to form a coolant flow path. A coolant flowing through the inlet header portion 60b into the flow path in the tubular element will travel towards another end thereof, and makes one U-turn before coming back into the outlet header portion 60c. These tubular elements thus build the "one-sided header" stack type evaporator which are employed widely in this field. The "one-sided header" structure is somewhat disadvantageous in that due to the U-turn which the coolant makes, it cannot flow evenly through the tubular elements but flows in an offset manner. This will inevitably reduce the effective heat transfer area of each tubular element.
Therefore, another type of evaporator which is of the "both-sided header" structure has been proposed and used in certain cases. This evaporator comprises the tubular element each having the inlet header portion at its one end and the outlet header portion at its other end.
The prior art stack type evaporators, whether of the one-sided or both-side structure, employ such recessed ribs 70 which are distributed over each core plate 60 as shown in FIG. 11. Those ribs are intended to render turbulent the coolant stream within the tubular elements so as to improve the heat transfer. In detail, many recessed ribs 70 protrude inwardly of two dish-like core plates 60 which are secured one to another at their peripheries to construct each tubular element ( see for example Japanese Utility Model Publication Sho. 56-6847 and ibid. 63-33100 ).
However in use of those stack type evaporators, water which is condensed on the surfaces of the tubular elements and fin members will stay within the recessed ribs 70. An angle of contact ( hereinafter simply referred to as "contact angle" ) of each waterdrop and the surface of tubular element or fin member, to which the water drop sticks, is so large as making it difficult to smoothly drain the condensed water. As a result the condensed water staying within air paths, which are each formed between the tubular elements and through the fin member interposed therebetween, will be scattered to fly into an automobile compartment to thereby spoil the air-conditioned comfortableness thereof. Further, the air paths get mildewed due to the sticking condensed water, and a bad smell of mildew or mold will render unpleasant the air stream which may unintentionally flow into the compartment.
A prior art invention disclosed on the Japanese Patent Publication Sho. 60-45776 was made to resolve such a problem. According to this proposal, the tubular elements and fin members are covered with a hydrophilic surface coating. The hydrophilic coating reduces the contact angle between the surface of tubular element or fin member and the waterdrop. Consequently, the condensed water forms on the surfaces a thin layer which will decrease the air flow resistance along the surface, and the thin layer does not stay thereon but is drained smoothly to resolve the problem of flying waterdrop.
As described in the Patent Publication Sho. 60-45776, a water glass-based coating has been preferred as the hydrophilic coating. A smell inherent in this water glass coating itself is however not pleasant, and spoils the air-conditioned automobile cabin into which the outer air stream inevitably flows. Thus, such a prior art coating is not free from an essential problem.
Another Japanese Patent Publication Sho. 61-39589 or Patent Laying-open Gazette Hei. 3-49944 discloses, on the other hand, another proposal which employs a polyamide resin as the component of hydrophilic coating in place of the water glass. Although polyamide resin coating does not emit such a bad smell as water glass, it fails to cause the adherent water to form a sufficiently thin layer. Thus, the air flow resistance through the evaporator is comparatively high, the condensed water is difficult to drain, and the problem of waterdrop flying is not resolved.
On the other hand, it must be noted of a certain drawback caused by the recessed ribs. Even if the adherent state and flowability of condensed water were improved with the hydrophilic coating of the outer surface of the evaporator, the condensed water stays in the outer recesses of the ribs 70 which are formed separately in a scattered manner over the outer surface of each tubular element. Drainage of condensed water is not improved to a satisfactory degree, and the waterdrop flying and other secondary problems remain unresolved. Further, due to the staying condensed water, the tubular elements are likely to get mildewed. The mold or mildew will give off a bad smell into an automobile cabin and makes it unpleasant.
Therefore, the present applicant has proposed in its Japanese Patent Application Hei. 1-223685 ( see Patent Laying-open Gazette Hei. 3-87595 ), not to rely upon any hydrophilic coating, but to give the tubular elements a revised shape such that the drainage of condensed water is improved to prevent the waterdrop from flying and the bad smell is shut out.
According to this prior art structure of the evaporator, each tubular element is formed with a plurality of recessed ribs. Those ribs extend straight and in parallel with one another from an upper header portion to a lower header portion of said element. Because such ribs do function as drainage canals, the condensed water flows downwards to be discharged from the lower header portion. Thus, the condensed water sticking to the surface of tubular elements is removed smoothly through the recessed ribs, providing an improved property of "water repelling".
In the condenser of this type, corrugated fins are each interposed between two adjacent tubular elements and are so highly water-bearing that the condensed water cannot move readily from the fins onto the recessed ribs on tubular elements. Although the condensed water on the fins at their outer regions or outer ends adjoined to the tubular elements will readily move onto the latter and into their recessed ribs, the condensed water present deep in concaves of the corrugated fins will tend to stay there due to a strong surface tension. In other words, some fractions of the condensed water on the fins are not necessarily removed through the recessed ribs of the tubular elements.
As a result, the drainage of condensed water remains not improved to a satisfactory degree, also failing to resolve the problems of waterdrop flying and bad smell, which is emitted for example from the mold in the remaining and sticking water.
It may be natural to employ the hydrophilic coating composed of water glass or synthetic resin also for the tubular elements which are formed with the straight recessed ribs serving as the improved drainage canals.
However it will not be possible here too to avoid the already described problem that on one hand the water glass hydrophilc coating is unpreferable due to its stinking smell, and on the other hand the prior art resin coating cannot improve the adherent state and flowability of the condensed water. The condensed water will not move smoothly to such recessed ribs even though they are of the shape of straight drain canals, thus rendering the drainage unsatisfactory and failing to resolve the problem of waterdrop flying.